5.1.6 Break Conditions

The simplest sort of breakpoint breaks every time your program reaches a
specified place. You can also specify a condition for a
breakpoint. A condition is just a Boolean expression in your
programming language (see Expressions). A breakpoint with
a condition evaluates the expression each time your program reaches it,
and your program stops only if the condition is true.

This is the converse of using assertions for program validation; in that
situation, you want to stop when the assertion is violated—that is,
when the condition is false. In C, if you want to test an assertion expressed
by the condition assert, you should set the condition
‘! assert’ on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not need them,
since a watchpoint is inspecting the value of an expression anyhow—but
it might be simpler, say, to just set a watchpoint on a variable name,
and specify a condition that tests whether the new value is an interesting
one.

Break conditions can have side effects, and may even call functions in
your program. This can be useful, for example, to activate functions
that log program progress, or to use your own print functions to
format special data structures. The effects are completely predictable
unless there is another enabled breakpoint at the same address. (In
that case, gdb might see the other breakpoint first and stop your
program without checking the condition of this one.) Note that
breakpoint commands are usually more convenient and flexible than break
conditions for the
purpose of performing side effects when a breakpoint is reached
(see Breakpoint Command Lists).

Breakpoint conditions can also be evaluated on the target's side if
the target supports it. Instead of evaluating the conditions locally,
gdb encodes the expression into an agent expression
(see Agent Expressions) suitable for execution on the target,
independently of gdb. Global variables become raw memory
locations, locals become stack accesses, and so forth.

In this case, gdb will only be notified of a breakpoint trigger
when its condition evaluates to true. This mechanism may provide faster
response times depending on the performance characteristics of the target
since it does not need to keep gdb informed about
every breakpoint trigger, even those with false conditions.

Break conditions can be specified when a breakpoint is set, by using
‘if’ in the arguments to the break command. See Setting Breakpoints. They can also be changed at any time
with the condition command.

You can also use the if keyword with the watch command.
The catch command does not recognize the if keyword;
condition is the only way to impose a further condition on a
catchpoint.

condition bnumexpression

Specify expression as the break condition for breakpoint,
watchpoint, or catchpoint number bnum. After you set a condition,
breakpoint bnum stops your program only if the value of
expression is true (nonzero, in C). When you use
condition, gdb checks expression immediately for
syntactic correctness, and to determine whether symbols in it have
referents in the context of your breakpoint. If expression uses
symbols not referenced in the context of the breakpoint, gdb
prints an error message:

No symbol "foo" in current context.

gdb does
not actually evaluate expression at the time the condition
command (or a command that sets a breakpoint with a condition, like
break if ...) is given, however. See Expressions.

condition bnum

Remove the condition from breakpoint number bnum. It becomes
an ordinary unconditional breakpoint.

A special case of a breakpoint condition is to stop only when the
breakpoint has been reached a certain number of times. This is so
useful that there is a special way to do it, using the ignore
count of the breakpoint. Every breakpoint has an ignore count, which
is an integer. Most of the time, the ignore count is zero, and
therefore has no effect. But if your program reaches a breakpoint whose
ignore count is positive, then instead of stopping, it just decrements
the ignore count by one and continues. As a result, if the ignore count
value is n, the breakpoint does not stop the next n times
your program reaches it.

ignore bnumcount

Set the ignore count of breakpoint number bnum to count.
The next count times the breakpoint is reached, your program's
execution does not stop; other than to decrement the ignore count, gdb
takes no action.

To make the breakpoint stop the next time it is reached, specify
a count of zero.

When you use continue to resume execution of your program from a
breakpoint, you can specify an ignore count directly as an argument to
continue, rather than using ignore. See Continuing and Stepping.

If a breakpoint has a positive ignore count and a condition, the
condition is not checked. Once the ignore count reaches zero,
gdb resumes checking the condition.

You could achieve the effect of the ignore count with a condition such
as ‘$foo-- <= 0’ using a debugger convenience variable that
is decremented each time. See Convenience Variables.